ghostchar 👻

Detect, encode, and decode invisible / dangerous Unicode characters — the ones used for ASCII smuggling (hidden LLM instructions), Trojan Source attacks (CVE-2021-42574), and zero-width steganography.

One shared engine, two surfaces:

Package	What it is
@ghostchar/core	Dependency-free TypeScript engine: detect, encode, decode (tags / variation-selector / zero-width).
ghostchar	CLI: detect invisible characters, encode / decode Unicode Tag payloads.
ide-ghostchar	Editor extension: inline highlights, diagnostics, hover, quick-fixes, paste guard. Publishable to the VS Code Marketplace and Open VSX.

Defensive & research tool. encode reproduces known smuggling techniques so detectors, tests, and demos have realistic payloads.

Detected categories

unicode-tags · bidi-control · zero-width · variation-selector · invisible-operator · non-standard-space

Every flagged character is treated the same — detected, highlighted, and warned about. There are no severity tiers.

Develop

pnpm install
pnpm build      # builds core, then bundles cli + extension
pnpm test       # runs core unit tests (vitest)

CLI usage

ghostchar detect "src/**/*.ts"                # exit 1 on any finding (CI gate)
ghostchar detect file.txt --json > out.json   # machine-readable output
ghostchar detect . --sarif --no-fail > ghostchar.sarif   # GitHub code scanning
ghostchar detect "src/**/*.ts" --no-fail      # report only, always exit 0

# encode: --scheme tags (ASCII only, default) | variation-selector | zero-width
ghostchar encode "hidden text"                            # ASCII via Unicode Tags
ghostchar encode "こんにちは" --scheme variation-selector  # any Unicode (Japanese, emoji)
ghostchar encode "秘密" --scheme zero-width                # any Unicode via zero-width bits
echo "secret" | ghostchar encode                          # reads stdin when no arg

# decode tries all schemes by default (or pass --scheme)
ghostchar encode "こんにちは" --scheme zero-width | ghostchar decode -
ghostchar decode file.txt                     # decode any hidden payload in a file

Extension (debug)

Open packages/ide-extension and press F5, or build a .vsix:

pnpm --filter ide-ghostchar package

Educational Purpose

This tool is built for security research and education. Understanding these encoding techniques helps defenders build better detection systems. Use responsibly. @